Known injection-moulding devices, for producing objects of different kinds, usually comprise two main mould parts for defining a cavity. A first, fixed part has an injection nozzle, injecting the liquid moulding compound, typically a warm resin such as polycarbonate, into the cavity. A second mould part is movable in relation to the fixed mould part, such that it can be moved to open the cavity. The volume of the mould cavity, for forming the object, may be arranged in either or both of the mould parts. In so-called injection compression injection-moulding devices there may also be a third part defining the cavity.
When forming an object, liquid resin from an extruder is injected into the mould cavity through the nozzle when the two mould parts are in a closed or nearly closed state. As soon as an adequate amount of resin has been injected, the mould parts may be closed by pushing the movable part towards the fixed part, which allows a fine structure to be formed on an object surface, the structure being an inverse of a structure formed on one of the mould parts. E.g. if a CD (Compact Disc) is produced, this structure comprises a digital pattern intended, after several post-processing steps, to be readable by means of a laser pick-up system.
The mould parts are then cooled, usually by means of cooling water flowing in integrated cooling ducts, such that the resin in the cavity is solidified. Subsequently, the mould parts are separated by retracting the movable mould part, and the finished object can be removed.
The injected resin should be warm enough to allow the resin to completely fill the cavity and any desired surface structure to be reproduced before the resin reaches its glass transition region. This imposes limitations upon the temperature of the moulded resin when injected into the cavity as well as upon the temperature of the cooling water. When injected into the cavity, the resin temperature must be substantially higher than the upper range of the glass transition region. When the injection phase is completed and the cooling phase follows, at least some parts of the mould need be cooled from a starting temperature which is quite high. This is done with cooling water, the temperature of which must not be so low that the resin is solidified too early. The cooling water temperature may be 115° C. in a typical DVD-R manufacturing process. Therefore, the time needed for cooling is quite long. The total cycle time needed to injection mould e.g. a DVD-R (Digital Versatile Disc, Recordable) substrate is about 3 seconds. The cooling water flows continuously, and its temperature is carefully regulated.
Attempts have been made to reduce the total cycle time by providing auxiliary heating to the mould parts. When the mould parts are not just heated by the in-flowing hot resin, the temperature of the cooling water can be lowered, since the auxiliary heating compensates, during the injection phase, for the lower cooling water temperature. It can therefore still be ensured that the resin fills the cavity completely and that any surface structure is replicated before the resin reaches its glass transition region.
Therefore, due to the lower cooling water temperature, the cooling time needed to ensure that the resin has been solidified may be reduced.
GB, 2081171, A describes an injection moulding machine where the mould parts are heated by induction heating. An inductor is placed in a mould part in the vicinity of the mould cavity surface and is fixed in a hard epoxy resin. A high frequency oscillator is connected to the inductor, such that the inductor heats the mould part during production.
U.S. Pat. No. 4,563,145, A describes a moulding block for manufacturing flat information carriers from thermoplastic material. The moulding block comprises a glass plate, wherein a copper pipe is embedded. The copper pipe can be used both as a cooling channel and an inductive coil. On top of the glass plate, a ferromagnetic material layer is placed, which absorbs some of the energy emitted from the coil. On top of the ferromagnetic layer, a copper layer is placed, having high thermal conductivity. The copper layer conducts the generated heat to a surface layer in the mould part.
Thus, injection moulding machines with inductively heated mould parts have been known for some years, but have not come to a widespread use, as far as the inventors know. There is a trade-off between increased complexity and decreased cycle times. Known solutions have not been efficient enough to compensate for the increased complexity.